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You’re starting to wonder just how long it will take before you finally reach your destination, when suddenly a little yellow light on the dash winks on: Your DPF filter needs attention. Great… Just what you need. A problem to deal with. And just what on earth is a “DPF” anyway?

By Jack Roberts

First off, there’s no need to panic. That’s because the truck isn’t in any trouble. And neither is the engine, for that matter. That little light winking on is just letting you know that a normal function in the life of an EPA 2007 or 2010 diesel engine has to take place. And the good news is that you’re probably going to have to do little to make that routine piece of maintenance happen. In fact, DPF lights are such a normal, non-event, some experts, like David McKenna, director of powertrain sales and marketing for Mack Trucks, argue that there’s not really a good reason to put a DPF warning light on the dash of a truck. “We’re worrying drivers for no reason,” he says. “DPF functions are really invisible and should be a total non-event for a driver.”

Here’s a quick recap on DPFs:

What is a DPF?

The DPF light (lower right) on the dash board of Kenworth’s T680.

“DPF” is shorthand for “diesel particulate filter.” It’s an exhaust filter, just like a fuel or oil filter. It’s set up in the exhaust system of your truck, always upstream from the diesel fluid injector SCR catalyst chamber. One of the main provisions of the Environmental Protection Agency’s 2007 diesel emissions regulations was the reduction of diesel particulates in diesel exhaust smoke and that requirement carried over into the 2010 regulations we’re operating under today. So any truck manufactured after January 1, 2007, is equipped with a DPF.

Smoke is smoke. It just floats away, right?

Strange as it seems, there’s actually a lot of heavier-than-air stuff in diesel smoke. You and I would call it “soot.” The EPA calls it “diesel particulates.” They are tiny bits of soot leftover from the combustion process, comprised of fine and ultra-fine particles. These particles can contain elemental carbon with adsorbed compounds such as organic compounds, sulfate, nitrate, metals and other trace elements.

So, obviously, the diesel particulate filter traps all these little bits of soot before they can escape out into the atmosphere.

Exactly. But over time, these filters fill up and start to clog – just like any filter on your vehicle does.

Does that impact my vehicle or engine performance?

Yes. Much like a clogged catalytic converter on your car, a full DPF interferes with efficient exhaust flow out of the engine and if left untreated, can lead to compression or combustion problems.

What does it look like and where is it located?

Actually, it’s not a spin-on type filter like an oil filter or a cartridge-type like a fuel filter – and they work differently, as well. In most cases, the DPF burns off accumulated soot through either a “passive” or “active” regeneration. In a passive system, the engine’s onboard computers track certain variables such as exhaust NOx emissions and temperatures and heat up the DPF to oxidize (burn off) the soot as part of a normal duty cycle. In an active system, diesel fuel is directly injected into the exhaust stream, usually right behind the turbocharger. This fuel evaporates and coats the DPF catalyst causing a chemical reaction (lots of heat) and this action oxidizes any remaining soot, leaving a residual amount of ash. Either event is likely the reason that little yellow dash light came on.

So I really don’t have to worry about it at all?

Not really, but there is another reason that light may be on: It may be time to have the DPF cleaned. At around 400,000 miles, the system has accumulated enough ash to fill most of the DPF canister.

Isn’t that what the regeneration process does?

A DPF cleaning machine in action at one of Mack Trucks’ testing and validation facilities. The machines can cost as much as $75,000, which is why cleaning fees are so high.

The regeneration process burns off accumulated soot in the DPF. But it does not clean out the actual filter media that traps the particulate matter as exhaust flows through the DPF. And – as noted – the regeneration process will not burn off all the ash. Eventually it will collect in the canister and fill it up. This normally happens around the 400,000-mile mark for heavy-duty truck engines. But, if you have a turbocharger or fuel injector failure, the filter media could become saturated before those mileage intervals are met. So the little yellow DPF light on the dash might be an indication of a bigger problem, depending on the mileage since the last cleaning.

Is this an emergency situation? How soon do I need to get the DPF in for servicing?

It’s not immediate issue if you’re around the 400,000-mile mark. Usually you’ll know you’re getting close because the regenerations will be getting closer and closer together. If you’re getting more and more lights and regenerations well before the 400,000-mile mark, you may have a bad injector or turbo. In any event, remember that if you wait too long, your vehicle performance will fall off noticeably as the engine de-rates due to excessive exhaust back pressure. So it pays to get the DPF looked at as soon as possible.

How long does a cleaning take?

There are two types of cleaning and both need to be done by certified DPF technician. If you need to change out the DPF – usually because it is chipped, cracked, burned or melted – it takes about 2 1/2 hours, depending on vehicle or engine make. If your DPF is in good shape and just needs to be cleaned, you’re probably looking at about 4 to 5 hours.

How much does it cost?

If you’re replacing the DPF, you’re probably looking at about $1,200. Getting the unit cleaned will run you about $500.

Can I just do it myself?

You could, but it’s not recommended. The DPF element is made of ceramic material and is very fragile. It has to be treated with the upmost care. You sure don’t want to drop it because a non-core replacement DPF can cost as much as $4,500!

Editor’s note: The following companies contributed to this story: Mack Trucks, Cummins, Detroit and Navistar.